Paris ? After Brussels, President Obama’s strategy of gradually degrading the Islamic State looks terribly risky. And much too slow.

Yes, the Islamic State has lost around 40 percent of the territory it seized in Syria and Iraq, much of it retaken by Kurdish forces with U.S. air support. But it still holds the cities at its heart: Raqqa in eastern Syria and the major urban area of Mosul in northern Iraq. Right now, it appears unlikely that either will be liberated in the near term.

That cross-border territorial base enables the jihadis to retain their ideological appeal to alienated Muslim youths in Europe and elsewhere. That base also permits the Islamic State to maintain its fiction that it has resurrected a caliphate. It allows it to train fighters with European passports who can return home and wreak carnage.

Moreover, the fear generated by these attacks is stoking a wave of right-wing populism in Europe that threatens to undermine America’s most important alliances — the European Union and NATO.

Quicker action is needed

None of this excuses the glaring European failure to share intelligence, or act on information that might have prevented last week’s tragedy. But the Brussels disaster forces the question:

Should the United States be moving more quickly to upend the caliphate, before the Islamic State sinks deeper roots into Europe and North Africa — and spreads to America?

After spending two weeks in Iraq and Syria, my answer is yes. But I don’t mean send in the Marines.

Indeed, it is best to look at what won’t work before laying out the one plausible (but risky) option that could do the caliphate in.

For starters, let’s scratch the idiotic post-Brussels panaceas offered by Donald Trump (more torture) and Ted Cruz (“patrol and secure” Muslim American neighborhoods). Both would create more recruits for the Islamic State than they would deter.

Retaking Mosul delayed

The complexity of the Syrian front has led many to focus first on the Iraq option. U.S. troops have been retraining the Iraqi army that collapsed in 2014 when the Islamic State invaded Mosul. Early hopes were that Mosul could be liberated in 2015, but that goal was pushed back to 2016 and even that date is now in serious doubt.

Speaking at a forum held by the American University of Iraq in Sulaimani, the special presidential envoy for countering the Islamic State cautioned against expectations for a speedy liberation of Mosul. “Everything now is trending the right way, but this will be a long haul,” said Brett McGurk in Kurdistan. “This is going to be longer than people want.”

The reasons for the delay: A weak and beleaguered Iraqi prime minister is being pressed by Iranian-backed militia leaders to let their Shiite fighters lead the offensive on Mosul — a predominantly Sunni city. This would risk an outbreak of sectarian slaughter.

Political infighting has also delayed the raising of a Sunni tribal force of 15,000 men from northern Iraq to join the army. This Sunni component is essential to reassure Mosul civilians that the anti-Islamic State force won’t take revenge on them.

“Fighting between politicians in Baghdad while we are fighting ISIS puts a stone on the way of Iraq,” I was told by Gen. Najm al-Jabouri, commander of the operations center tasked with organizing the Mosul offensive. “We don’t have enough troops, enough equipment,” he told me at his headquarters in Makhmour.

Retraining of Iraqi army troops is slow. Jabouri says he has three brigades but could use eight to 13. These deficits mean that the Mosul offensive will likely take place in stages; any move to retake the urban core probably won’t happen until 2017.

This harsh truth forces us to reexamine whether there is any prospect of an earlier liberation of Raqqa, and who could provide the troops.

The easy (but incorrect) answer is: Let the Russians do it with their Assad regime ally. Syrian government troops, backed by Russian air power (yes, Vladimir Putin is still active in Syria) are on the verge of retaking Palmyra, which lies along a crucial route to Raqqa.

Retaking Raqqa

But it would be a stretch too far to expect the unsteady Syrian military to take Raqqa. Nor do Moscow or Damascus have a strategic reason to do so. (The continued existence of the Islamic State is used by Russia to justify maintaining Bashar al-Assad in power.)

Which brings us to the only fighters willing and able to retake Raqqa from the Islamic State if a host of political obstacles didn’t prevent them: the Syrian Kurds.

Syrian Kurdish fighters, backed by U.S. air power, have already driven the Islamic State out of large swathes of eastern Syria. “We think Raqqa should be retaken as soon as possible,” said Salih Muslim, co-president of the Syrian Kurds’ Democratic Union Party (PYD), speaking at the Sulaimani forum. “We can stand against ISIS and break down the myth of ISIS.”

He’s probably right. With greater U.S. support, and an effort to enlist more Sunni tribal fighters to join the Kurds, they might well take Raqqa. Muslim says they would give this Sunni city local rule within their self-declared federal region of Rojava and North Syria. He says the Kurds would ensure that the Islamic State 2.0 would not emerge in the liberated area.

Work harder on Turkey

But Turkey, at war with its own rebel Kurds, violently opposes this idea.

Washington should try harder to persuade Turkey’s mercurial President Recep Tayyip Erdogan to renew talks with Kurdish rebels in both Turkey and Syria, but he doesn’t seem open to reason.

Moreover, it’s far from clear that Raqqa’s Sunni Arab population would accept living under Kurdish overlordship. “This could become a real incendiary issue between Kurds, Arabs and Turks,” says Joshua Landis, the University of Oklahoma’s noted Syria expert.

Yet if the goal is to destroy the Islamic State caliphate more quickly, helping the Kurds of Syria take Raqqa may be the only option, one that must at least be seriously examined. The longer the Islamic State keeps its territorial base in Raqqa and Mosul, the more its threat to the West will grow.

— Trudy Rubin is a columnist and editorial-board member for the Philadelphia Inquirer.